The present invention relates to a variable frequency synthesizer which is used as a local oscillator of a spectrum analyzer or a test signal generator for producing signals of frequencies above the VHF band, and more particularly to a variable frequency synthesizer which is excellent in SSB phase-noise characteristic.
The variable frequency synthesizer of the present invention is used singly as a variable frequency synthesizer in itself but can be used as a part of a frequency synthesizing system as well.
In a prior art signal generator, as shown in FIG. 1, a signal from a variable frequency synthesizer 20 is applied via a terminal 18 to a frequency mixer 19, wherein it is frequency mixed with the output of a voltage-controlled variable oscillator (VCO) 21. The output of the voltage-controlled variable oscillator 21 is converted by the frequency mixer 19 to a signal of a lower frequency, which is provided to a phase comparator 23, wherein it is phase compared with a reference signal of a frequency f.sub.2 from a variable reference signal generator 22. The phase-compared output is applied to an adder 24, wherein it is added to a DC signal from a coarse frequency setting circuit 25, and the added output is provided as a frequency control signal to the voltage-controlled variable oscillator 21. For example, the variable frequency synthesizer 20 varies, by steps of 200 MHz, its output signal frequency ranging from 1800 to 3800 MHz, the variable reference signal generator 22 varies, by steps of 1 KHz, its output signal frequency ranging from 200 to 300 MHz and the coarse frequency setting circuit 25 sets an approximate value of the frequency to be created, that is, a value of the higher-order digits of the frequency value to be generated. Thus the output frequency f.sub.0 of the VCO 21 can be varied in the range of between 2000 to 4000 MHz by the steps of 1 KHz, and the output of such a frequency is provided to an output terminal 41.
As the variable frequency synthesizer 20, a frequency synthesizer is employed, which has heretofore been arranged, as shown in FIG. 2. A reference signal of a frequency f.sub.s is applied from a reference signal source 11 to each of frequency multipliers 42 and 43. The frequency multiplier 42 provides a signal of a frequency n.sub.1 f.sub.s, whereas the frequency multiplier 43 produces n.sub.2 signals of frequencies f.sub.s, 2f.sub.s, 3f.sub.s, . . . and n.sub.2 f.sub.s. The output signal of the frequency multiplier 42 and the output of a voltage-controlled variable oscillator 44 are applied to a frequency converter 45, wherein the output frequency of the variable oscillator 44 is lowered. The output of the frequency converter 45 is phase compared, by a phase comparator 46, with the output of the frequency multiplier 43. The phase-compared output is added, by an adder 47, to the DC output of a coarse frequency setting circuit 48. The added output is applied to the variable oscillator 44 to control its output frequency. By that one of the output signals of the frequency multiplier 43 which corresponds to the set value of the coarse frequency setting circuit 48 is effected loop control for the variable oscillator 44.
Assuming that the oscillation frequency f.sub.s of the reference signal source 11 is 100 MHz and the multiplication factors n.sub.1 and n.sub.2 are 18 and 20, respectively, the output frequency of the frequency multiplier 42 will by 1800 MHz. By suitable setting of the coarse frequency setting circuit 48, the oscillation frequency of the variable oscillator 44 can be varied in the range of 1800 to 3800 MHz at intervals of 100 MHz. At this time, the output frequency of the frequency converter 45 ranges from 0 to 2000 MHz.
As described above, the conventional variable frequency synthesizer 20 employs frequency multipliers which frequency-multiply the reference signal so as to derive signals of higher frequencies from the reference signal. In general, the reference signal has an instantaneous phase variation, which is also multiplied when the frequency is multiplied by a frequency multiplier. As a result, the output signal of the conventional frequency synthesizer is low in frequency purity.